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Ensure that flow operators propagate the cancellation exceptions #4038

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merged 3 commits into from Feb 15, 2024

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dkhalanskyjb
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Before this change, it could happen that some size-limiting operators upstream swallowed the requests to limit the flow size emitted by the operators downstream.

This could cause onCompletion calls between these operators to incorrectly report that the flow was not in fact limited by the downstream operators.

Additionally, in the presence of additional size-limiting operators in the chain, first and single and their variants could exhibit incorrect behavior where emitting a value from onCompletion would overwrite their output.

Fixes #4035

Before this change, it could happen that some size-limiting
operators upstream swallowed the requests to limit the flow size
emitted by the operators downstream.

This could cause `onCompletion` calls between these operators to
incorrectly report that the flow was not in fact limited by the
downstream operators.

Additionally, in the presence of additional size-limiting operators
in the chain, `first` and `single` and their variants could exhibit
incorrect behavior where emitting a value from `onCompletion` would
overwrite their output.

Fixes #4035
@qwwdfsad
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Could you please double-check my understanding?

At first glance, the behavioural difference in this snippet

flowOf(1)
    .take(1)
    .onEach {  } // 1
    .onCompletion { println("??? $it") }
    .first()

with 1 being commented out or not just doesn't make any sense. I tried to figure it out by just looking at the commit message, and it was unclear, thus decided to understand the root cause before looking at the solution to avoid spoiling the fun review.

It seems to me that the root cause is the following:

  1. take uses FlowCollector that it receives in an argument as an ownership token, and it checks the ownership in catch block
  2. The problem is that FlowCollector is not unique to the take itself and, in fact, depends on the downstream operator. It might be a collector from onEach, or it might be an onCompletion collector.
  3. onCompletion collector, which is collectWhile collector, throws an AbortFlowException and uses itself as an ownership token.
  4. According to 2, ownership check in take might pass or might not pass, leading to the observed behaviour

So the root cause is straightforward -- we cannot use FlowCollector from an argument as an ownership token

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Nice one!

@dkhalanskyjb dkhalanskyjb merged commit d0dabb9 into develop Feb 15, 2024
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@dkhalanskyjb dkhalanskyjb deleted the dk-fix-AbortFlowException-ownership branch February 15, 2024 09:31
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2 participants